Help, Dark Blonde Hair darker at the front of head! Any one else got the same problem

[Golden_Valkyrie]

I'm a natural dark blonde, and have had my roots touched up using a 12.1 highlift colour for about five years professionally. It's been fine up until about a year ago when the colour stopped taking as well on my roots. I'd considered lots of factors including development time but both me and my hairdresser were both bemused as to this. I think it is down to the front third of my parting being roughly a shade darker blonde than the back. This is causing unsightly root touch ups.
Does anyone else have the same problem or am I just odd?